1. Field of the Invention
This invention relates to methods which include the use of radiation polymerisation for the manufacture of fibre-resin intermediates which are suitable for use as, or use in, mouldable fibre reinforced polymer compounds.
2. Description of the Related Art
One of the most common approaches to the production of reinforced thermoplastic composite intermediates for subsequent moulding processes, such as injection moulding, is extrusion compounding. Chopped fibres and additives are usually dispersed within a polymer melt via a screw extruder to produce a continuous strand or other shape which is then subsequently cooled or solidified and chopped to produce a moulding compound such as a granule or pellet. This process has the disadvantage of only producing short fibre lengths in the mouldable intermediate product since fibre feedstock is already chopped (typically 3-6 mm) and the conventional extrusion process is known to impart significant fibre breakage and hence reduce fibre lengths further. In addition, high temperatures are needed to melt the polymers to a suitable viscosity for effective fibre wetting or impregnation. Good control of fibre-polymer adhesion is not always easily achievable. Furthermore the process is not well suited to efficient incorporation of certain additives which may be desirable eg liquids (which are inconvenient to feed into an extruder) and/or materials for controlling the fibre-polymer interface, which would usually be dispersed randomly within the fibre-polymer mixture (in the extruder) rather than deliberately located on or near to the fibre. Introduction of a coating around fibres, or other routes to influence the fibre-resin interphase region, in fibre-polymer composites is not convenient, unless carried out by the fibre manufacturers.
There are potential advantages, such as an improved balance of mechanical properties (eg toughness or impact resistance and strength/modulus) in achieving longer fibre lengths in a final moulded component Longer fibres in the mouldable intermediate can often translate into longer retained fibre lengths in the final moulded component. In addition, good fibre impregnation and/or adhesion, or some control over bonding between the fibres and polymer in the fibre--polymer interphase region is often desired for property optimisations.
Several routes to the manufacture of long fibre reinforced thermoplastic moulding compounds are known and a key issue is the achievement of good impregnation of the continuous fibres with the thermoplastic polymer. Such inpregnated continuous fibres are usually chopped to a desired length. Example processes for impregnation of continuous fibre reinforcement include hot-melt impregnation, solvent based impregnation, fluidised bed/powder impregnation, and comingling of thermoplastic fibres with reinforcing fibres. Processes are known in which a fibre reinforced composite structure is produced by pulling continuous fibres through a resin bath containing a low viscosity liquid formulation to impregnate the fibres. The formulation is subsequently cured by heat in a heated forming die to a thermoset product. As such this product is not usually subsequently mouldable or thermoplastically processable. This is the well known technology of pultrusion as applied to thermoset composite products.
Variations on this process which essentially involve a hot-melt impregnation approach (`melt-pultrusion`) have been disclosed for the production of thermoplastic composites, see for example EP 056 703, U.S. Pat. No. 5,213,889 and related patents and also the article by S. J. Bowen & P. H. Johnson in Engineering Plastics Vol 4, No 3, pp 188-197 (1991).
In such melt-pultrusion processes thermoplastic melts are used for fibre impregnation and this often necessitates the use of high temperatures, even for many low molecular weight polymers, and can place some limitations on the maximum production speeds that can be used for achievement of good wetting of the fibres or filaments. In some cases the thermoplastic melt is delivered to a resin bath or impregnation head via a screw extruder. Thus, in such cases, continuous fibres are passed through an impregnation head attached to the output of an extruder or other equipment from which molten polymer can feed.
Although such processes do produce a long fibre reinforced thermoplastic intermediate which is subsequently mouldable there are inconveniences associated with the use of hot, molten thermoplastics including handling and degradation of polymer at the high temperatures required for good fibre wetting. In many cases relatively high polymer melt viscosities are experienced which limit line speed. Particular care is required to maintain a sufficiently low viscosity melt and careful control of heating and cooling effects is necessary since melt viscosities may become too high for good impregnation. The design, including temperature control, of spreaders or rollers or pins or other aids to maximise fibre impregnation can be particularly important when hot molten polymers are used for impregnation.
The use of a pultrusion process in the production of mouldable composite intermediates is also described in articles by H. Ishida and D. A. Zimmermann in Proceedings of the 47th Annual Conference, Composites Institute, The Society of Plastics Industry Inc, Feb. 3-6 1992, in paper 18-D and also in Polymer Composites, April 1994, Vol 15 No2 pp 93-100. In these articles a two part reactive resin formulation, which is of low viscosity during impregnation or fibre wetting, is used in a RIM (Reaction Injection Moulding)--pultrusion process. Because of the low viscosity of the resin formulation during impregnation, well wetted fibres are likely to be obtained. However, although His process overcomes problems of pot-life, which are common to reactive systems, special RIM equipment is necessary to keep components of the resin formulation apart until required, since reaction is rapid on mixing. Such special RIM equipment will also promote rapid, efficient mixing of the reactive components. In principle fibre reinforce thermoplastic intermediates can be produced by this process, only a relatively narrow, specific range of resin components is suitable.
Radiation processes are known in the production of certain composites. U.S. Pat. No. 4,092,443 and U.S. Pat. No. 4,666,954 describe the preparation of prepregs from a photopolymerisable compound mixed with a heat activated curable epoxy system (thermosettable). GB 2 003 896 describes the production of composites from press moulding of prepregs (resin impregnated fibres sheets or tapes prepared from continuous fibres) or of sheet moulding compounds (impregnation of ready chopped (dry) fibres within a sheet or layer) from a composition containing a heat activated thermosettable (epoxy) resin system, a photo-polymerisable resin and photoinitiator. The composites produced after moulding are thermosets and as such involve a reaction during the final moulding of a component. Furthermore, there is no use of pultrusion or of mouldable intermediates such as pellets or granules for injection moulding. In addition a specific resin composition based on Friedel Crafts resins is required.
WO 89/03761 describes the use of radiation curing to immobilise a fibre-resin composition which also contains an ambient temperature curable resin component. Manufacturing processes such as pultrusion and filament winding are described. However, the final composite products are thermoset and are produced after a second curing stage involving crosslinking reactions applied to the whole pultruded or filament wound structure. There is no reference to chopping or producing a mouldable intermediate such as a pellet or granule.
Radiation based pultrusion processes are reported in, for example, EP 0 290 849 which refers to an example of UV-pultrusion, although other variations are known. GB 2 245 893 A discloses a method for microwave pultrusion